1. Field of the Invention
The present invention relates to a magneto-optical recording medium having a recording layer made of a garnet, and particularly, to a magneto-optical recording medium having a garnet recording layer formed on an amorphous substrate such as a glass substrate. The present invention also relates to a method for producing such a magneto-optical recording medium.
2. Description of Related Art
A garnet thin film containing yttrium (Y) or a rare-earth element and bismuth (Bi) substituting part of these elements is regarded as an attractive material for forming a high-density magneto-optical recording medium because the film is able to provide large Faraday rotation with respect to visible light within a short-wavelength region, and to exhibit excellent perpendicular magnetic properties.
On an experimental basis, a substrate made of a gadolinium-gallium-garnet (GGG) crystal is used as a substrate on which such a garnet thin film is to be formed, and a bismuth-substituted garnet film having good crystallographic properties is formed on the substrate. A GGG substrate, which is made of a single crystal and which has better thermal resistance than a glass substrate, enables the epitaxial growth of a garnet film at a relatively high temperature. If an ideal substrate, such as a substrate of a GGG crystal, is used, it is possible to obtain a magneto-optical recording medium having a low-noise bismuth-substituted garnet recording layer. However, GGG crystals are expensive, and cannot be readily used in practice as substrates of magneto-optical recording media.
A bismuth-substituted garnet film of the above-described type is usually formed by a sputtering method. However, a garnet film having good crystallographic properties cannot be directly obtained by depositing a film on an amorphous substrate, such as a glass substrate, by sputtering a film-forming material. To cope with this problem, the following process is conducted: a bismuth-substituted garnet film deposited on a glass substrate by a sputtering method is crystallized by subjecting the film to post heat treatment effected under certain conditions with which the glass substrate is prevented from softening. However, a garnet film resulting from this post heat treatment is composed of crystallites having irregular crystallographic orientations. As a result, a recording layer comprising the thus obtained bismuth-substituted garnet film entails the generation of great noise due to light scattering by crystalline boundaries. Noise generated from a magneto-optical recording medium falls into two categories: noise caused by light scattering such as above; and noise caused by non-uniformness of magnetic properties, more specifically, non-uniformness of Faraday effect. In this specification, the term "noise" refers to noise of the former category unless otherwise specified.
The present applicant has previously proposed in Japanese Patent Application No. 3-219695, a method comprising: depositing an amorphous film having a chemical composition corresponding to yttrium-iron-garnet (YIG) and expressed by a chemical formula Y.sub.3 Fe.sub.5 O.sub.12, that is, a YIG precursor film, on the surface of a glass substrate by a sputtering method; crystallizing the film by heat treatment; and depositing, on the crystallized underlayer, a bismuth-substituted garnet film expressed by a chemical formula of, for example, Bi.sub.2 DyGa.sub.0.8 Fe.sub.4.2 O.sub.12. The formation of the underlayer reduces the level of noise from the level obtainable with a bismuth-substituted garnet film directly formed on a glass substrate.
An underlayer of a garnet, such as a YIG underlayer, facilitates the growth thereon of a bismuth-substituted garnet film having relatively good crystallographic properties. However, the disclosure of the above-identified application states that the level of noise is not reduced if the YIG underlayer has poor crystallographic properties, and the underlayer must be made of a garnet having a chemical composition containing completely no bismuth, as in the case of the YIG film, or only a very small amount of bismuth. This is because if the YIG film has poor crystallographic properties, it is not possible to grow on the YIG film a bismuth-substituted garnet film having good crystallographic properties.
Also, according to the above disclosure, heat treatment at a temperature of not less than 700.degree. C. is necessary in order to crystallize an amorphous film having a chemical composition containing no bismuth, as in the case of the YIG precursor film, and rapidly heating the YIG precursor film with an infrared lamp is effective in order to prevent long-period heat-treatment at such a temperature from causing the glass substrate to soften and be deformed. The disclosure proposes heat treatment conditions in which temperature is raised to 700.degree. C. at a speed of 10.degree. C./second, and then retained at 700.degree. C. for 2 minutes.
As shown in FIG. 11, the level of noise (noise due to light scattering) evaluated by forming an aluminum reflecting film on a YIG underlayer crystallized in the heat-treatment conditions proposed in the above disclosure, is still relatively high. Specifically, this level of noise is approximately three times higher than the level of noise obtainable by forming an aluminum reflecting film directly on a glass substrate. Such a high noise level is higher than the acceptable upper limit of the level of noise generated from a magneto-optical recording medium, and is considered to be exhibited even when a recording layer of a bismuth-substituted garnet is deposited on the underlayer. The evaluation data suggests that, in view of practical use, it is important to prepare, for recording layer formation, an underlayer having a further reduced noise level. The noise data shown in FIG. 11 is obtained through measurement with a spectrum analyzer.
According to the results of observation using a differential interference microscope, a crystallized YIG film serving as the underlayer has a substantially completely smooth surface free from any recognizable unevenness. However, with a scanning electron microscope having a higher resolution, it has been observed that the YIG film is composed of crystallite grains having a grain size of several microns. Further, it has been found from observation using an atomic force microscope that the YIG film has an average surface roughness of not less than 40 .ANG..
As have been shown by the results of observation through the various types of microscopes, noise is generated due to light scattering even with a garnet film formed on a glass substrate and having a surface whose roughness is only of the order of 40 .ANG. which cannot be optically detected. Based on this fact, a structure of an underlayer that assures a smoother surface and a method for forming such a structure is considered to be essential.